I decided that a bit of chat about depth of field would be good. Don't ask me why, but it seemed like a good topic. So, here goes.
Depth of field defines that area of an image, from nearest to farthest that is in acceptable focus. OK, you say, but what do I care about that? Well, depth of field is one of the tools you can use to create images that isolate the subject—portraits for example—or that have everything in the photo showing sharp detail—think landscapes. Controlling depth of field is the way to achieve this.
Depth of field (DOF) is controlled by three things: focal length of the lens; size of the lens opening (f/stop); and distance between the camera and the subject. Here is how it works:
—the longer the focal length, the less the DOF (shorter=more DOF)
—the larger the lens opening (f/stop), the less the DOF (smaller=more DOF)
—the closer the camera is to the subject, the less the DOF (farther=more DOF)
—total DOF is approximately 1/3 in front of and 2/3 behind the actual point of focus
There is much debate about how the size of the film frame or digital sensor influences DOF. But that doesn't really matter, since any given lens, using the same settings, will create the same DOF regardless of what it is projecting on. All the jabber that pops up about "circle of confusion" and other techie terms doesn't change the physics of what hits the focal plane.
That first factor—focal length—is the one that is the biggest limiting factor when using compact digital cameras. Because the sensor is so small (mostly around 1/2" diagonal or less), the focal length of the lens is incredibly short to create similar angles of view of a 35mm film camera (everything uses comparisons to 35mm cameras). What would be a 50mm normal focal length on the film camera translates to around 7-8mm on the compact camera. And a 28mm wide angle lens for the film camera becomes something around 4.5-6mm. Focal lengths that short create tremendous DOF, so much so that it is often impossible to take a picture that has anything that isn't in focus. Even when zoomed out all the way, the compact camera that is showing you what a 200mm film camera lens would, the focal length is only around 25mm or so—still rather short in the realm of DOF. And few compacts have very large lens apertures.
So, what can you do if you are using a compact camera and want to take a nice portrait of someone with an out-of-focus background? First, use the portrait setting on the camera. It is designed to force the lens to its widest aperture (f/stop). The next thing to do is zoom out to the longest focal length (telephoto) the camera has. The final thing you will probably have to do is get as much separation as possible between the subject and the background. Even zoomed all the way out and opened all the way up, that compact camera lens is still going to be giving you more DOF than you want. If you can't separate the subject and the background, you can try moving closer and not zooming out quite as much. Be careful though, because if you get too close and use too short a zoom setting, you'll wind up making your subject's nose huge and spread all over their face.
There's not much to say about getting good landscape shots with a compact. They have plenty of DOF for that. But, if you are using a film camera or a digital SLR, it is much more problematic to achieve the DOF that keeps everything in focus. Use the shortest focal length you have available and stop that aperture way down. Just don't go too small or that tiny aperture may defract the light coming through and cause a blurry image. And when shooting landscapes, remember to focus on something about 1/3 of the way between the closest and farthest thing in the image that you want to be clear and sharp. You can't focus on the mountain in the distance and expect the bush 10 feet away to be in focus too.
To clear up a bit of confusion some of you may have, when talking about lens opening (aperture), you have to remember that, unlike everything else in photography, bigger is actually smaller. That is, the bigger the f/number, the smaller the lens opening. The f/number is actually the denominator (bottom number) of a fraction where the numerator (top number) is always one. It represents the relationship between the diameter of the opening to the focal length of the lens. So you can see that as that f/number gets bigger, the size of the opening gets smaller (e.g. f/2 = 1/2, f/8 = 1/8). Don't despair. The camera meter knows all about this and will take care of what needs taking care of when you make a change.